1. Field of the Invention
The present invention relates to a method for fabricating a semiconductor optical device, and more specifically it relates to a method for fabricating a semiconductor optical device suitable for application to an optical device such as semiconductor laser, a semiconductor optical amplifier, a semiconductor modulator, or a combination of such devices, which are used as major constituent elements in an optical communication systems.
2. Background of the Invention
Vapor phase epitaxial growth such as metal organic vapor phase epitaxial (MOVPE) growth feature superior control of film thickness, high uniformity and reproducibility of thickness, electrical and optical characteristics of grown films. So, they are of particular use in the fabrication of compound semiconductor electronic devices and optical devices. In crystal growth using MOVPE, if a mask is formed using a dielectric film on part of the substrate so as to inhibit crystal growth, the composition and growth rate of alloy semiconductor selectively grown on aperture region vary depending upon the mask width. In particular in the case of an MQW structure, not only the composition of each layer, but also the well layer thickness affects the bandgap, so that the bandgap is highly dependent upon the mask width. Using this effect, by varying the mask width in the light propagating direction, it is possible to fabricate integrated optical devices, for example, in which a laser and an optical modulator are grown simultaneously.
In semiconductor optical device structures, a buried structure is commonly used. The structure consists of mesa stripe active region that generates, amplifies or modulates laser light, and buried current blocking layers on the both sides of it. To achieve single-mode laser light output, the width of the active layer must be narrower than approximately 2 .mu.m. Therefore, to manufacture an optical device having a buried structure, it is necessary to have a step of forming a mesa stripe having a width of 2 .mu.m or less, and a step of forming a current stopping layer on both sides thereof.
Optical devices having a buried structure with current blocking layers using the above-noted selective growth are generally fabricated by using the following two methods. One method as follows. An active layer is grown by selective growth with a mask having a relatively wide aperture width of 10 .mu.m or greater. Then dielectric stripe having a width of 1 to 2 .mu.m is formed on the growth region by photolithography. Using this stripe as a mask, a mesa stripe is formed by etching, and then current blocking layers are grown. The other method is as follows. An active layer is grown by selective growth with a mask having an aperture width of 1 to 2 .mu.m. In this case, the mesa stripe with (111)B side facets is formed during growth. In the latter method, because an active mesa stripe structure having a width of 1 to 2 .mu.m is formed by this growth, it is not necessary to adjust the active stripe width before the current blocking layer growth using other process such as photolithography and etching. Therefore, high uniformity and reproducibility of the active mesa structure can be achieved. This method also has an advantage that no crystal fault caused by etching of semiconductor is generated. Because of the above-noted characteristics, by using this method, it is possible to manufacture an optical device featuring superior uniformity, reliability, and reproducibility. Using this method, however, in order to growth current blocking layers, a process step of forming a dielectric film only on the top of the mesa structure is needed. This process is extremely difficult using positioning by the normal photolithography process because the width of the top of mesa structure is only approximately 1 .mu.m. For this reason, a self-alignment process has been proposed (Sakata et al., Photo Tech. Lett., Vol. 8, No. 2, 1996). In this process, difference of the thickness of a SiO.sub.2 film formed by thermal CVD between the top and the sides of the mesa structure is utilized. By using this process, it becomes possible to form a dielectric layer on only the top of a mesa structure having a width of 1 .mu.m or less. This method requires the following steps, as shown in FIG. 4.
(a) Formation of a SiO.sub.2 403 by thermal CVD. PA1 (b) Removal of the SiO.sub.2 on both sides of the mesa by etching. PA1 (c) Formation of a resist strip 404 so as to cover the mesa 402, using photolithograhy. PA1 (d) Removal of the SiO.sub.2 from both sides of the mesa, using side etching. PA1 (e) Removal of the resist 404.
In the above-noted steps, at the (b) step it is necessary to precisely control the etching speed and time, and at the (c) step it is necessary to precisely control the positioning so that the mesa stripe is positioned in the approximate center of the resist stripe 404.
Thus, in order to form a dielectric layer on only the top of a mesa structure having a width of approximately 1 .mu.m, many process steps are needed, including steps requiring high precision, thereby making it difficult to improve the throughput.
The purpose of the present invention is to provide a novel method for fabricating a semiconductor optical device featuring greatly simplified process steps. It makes possible to use of a mesa structure including an active layer formed by selective growth as waveguide without etching, and to form a buried structure with current blocking layers without precise control of the etching rate and positioning during photolithography. It results superior uniformity, reproducibility and high throughput in manufacturing optical devices.